Many important enzymes in prokaryotes and eukaryotes are subject to covalent modification mediated by mixed-function oxidation systems. The modified enzymes are catalytically inactive. In the case of bacterial glutamine synthetase, the covalent modification "marks" the protein for subsequent proteolytic degradation by a protease which recognizes the modified protein. In order to determine the chemical nature of the modification, we isolated a cyanogen bromide peptide which contains the modified residue. The peptide from the modified protein has lost a single histidine residue and shows increased ultraviolet absorbance, when compared to the control peptide. The peptide is small (about five residues) and hydrophilic. Yeast enolase is also susceptible to the oxidative modification. After modification, the protein has lost one of nine histidine residues. Like bacterial glutamine synthetase, this modification also occurs in variable degree in enolase which has been purified to apparent homogeneity, suggesting that the modification occurs in vivo and is of physiologic significance. The oxidative modification "marks" proteins for degradation. The modification may also be important in host defense against microorganisms, in oxygen toxicity, and in the aging process.